The present invention relates to a liquid crystal display device and a method for fabricating the same, and more particularly, to a plasma addressed liquid crystal display device and a method for fabricating the same.
Development of plasma addressed liquid crystal display (LCD) devices is in progress for realization of large and thin flat displays. Plasma addressed LCD devices are liquid crystal display devices using a plasma cell for switching respective pixels. Size increase of this type of devices is easier compared with active matrix LCD devices using thin film transistors (TFTs), and therefore fabrication at low cost is possible. Plasma addressed LCD devices are disclosed in Japanese Laid-Open Patent Publication No. 1-217396 and No. 4-265931, for example.
A large problem to be overcome on plasma addressed LCD devices is that long-time maintenance of display quality is difficult. Plasma addressed LCD devices display images in the following manner. Plasma channels arranged in row are filled with ionizable discharge gas. Discharge plasma is generated in the plasma channels, to scan the plasma channels line-sequentially. In synchronization with this scanning, a voltage is applied to signal electrodes arranged in columns. In general, a plasma addressed LCD device emits ultraviolet rays during generation of discharge plasma. The emitted ultraviolet rays may degrade organic materials constituting a liquid crystal cell, such as liquid crystal molecules and alignment layers. By this degradation, the voltage retention decreases, for example, causing troubles such as local display exudation (irreversible display failure, indicating that the contrast ratio changes) and afterimage (reversible display failure, also called xe2x80x9cimage burn-inxe2x80x9d). This lowers the display quality of the LCD device.
In order to solve the above problem, Japanese Laid-Open Patent Publication No. 10-239671 discloses a plasma addressed LCD device that includes an ultraviolet transmission preventing layer formed on a sheet glass to prevent ultraviolet rays from a plasma cell from entering a liquid crystal cell.
There has been reported a technique of controlling the orientation of liquid crystal molecules by use of ultraviolet radiation for the purpose of improving the viewing angle characteristics of an LCD device. For example, Japanese Laid-Open Patent Publication No. 9-197384 discloses a plasma addressed LCD device of an axially symmetrically aligned micro-cell (ASM) mode that utilizes ultraviolet radiation during aligning processing. In the technique disclosed in this publication, ultraviolet rays including i-line (wavelength: 365 nm) are radiated from outside a plasma cell for stabilizing axially symmetrical orientation of liquid crystal molecules in the ASM mode LCD device.
Japanese Laid-Open Patent Publication No. 10-87859 discloses a technique of controlling the alignment direction (direction in which liquid crystal molecules are aligned) of an alignment film by irradiating the alignment film with linearly polarized ultraviolet rays. Japanese Laid-Open Patent Publication No. 10-148835 discloses a technique where a specific position of an alignment film is selectively irradiated with ultraviolet rays, to thereby selectively change the pretilt angle of liquid crystal molecules at the irradiated position and thus to widen the viewing angle.
The object of the present invention is providing a plasma addressed liquid crystal display device having wide viewing angle characteristics that can suppress or prevent deterioration in display quality due to ultraviolet rays from plasma channels and also can perform aligning processing using ultraviolet rays in a specific wavelength band, and a method for fabricating the plasma addressed liquid crystal display device.
The above object is attained by liquid crystal display devices of the first and second embodiments and the methods for fabricating the liquid crystal display devices, which are to be described below. The liquid crystal display device of the first embodiment of the invention includes: a substrate; a dielectric layer; a liquid crystal layer sandwiched by the substrate and the dielectric layer; a plurality of stripe-shaped electrodes formed on a surface of the substrate facing the liquid crystal layer to extend in parallel with a first direction; and a plurality of stripe-shaped plasma channels formed to face the plurality of electrodes with the liquid crystal layer and the dielectric layer therebetween to extend in parallel with a second direction different from the first direction, a plurality of pixel regions being formed in respective crossings of the plurality of electrodes and the plurality of plasma channels, wherein the dielectric layer selectively attenuates ultraviolet rays emitted from the plurality of plasma channels.
The dielectric layer of the LCD device of the first embodiment of the invention selectively attenuates ultraviolet rays emitted from the plurality of plasma channels. Accordingly, the dielectric layer can suppress or prevent organic materials such as liquid crystal molecules and alignment layers constituting a liquid crystal cell from degrading due to ultraviolet rays having a specific wavelength emitted by plasma discharge during use, and yet can transmit ultraviolet rays in other ranges. It is therefore possible to perform aligning processing and the like using ultraviolet rays allowed to pass through the dielectric layer.
Japanese Laid-Open Patent Publication No. 10-239671 mentioned above does not expect the case of positively utilizing ultraviolet rays, and thus does not disclose the idea of providing the ultraviolet transmission preventing layer with the function of transmitting ultraviolet rays in a specific wavelength range. Therefore, it is not possible to adopt the technique of controlling the alignment of liquid crystal molecules under ultraviolet radiation disclosed in Japanese Laid-Open Patent Publication No. 9-197384 mentioned above, for the fabrication of the plasma addressed LCD device having the ultraviolet transmission prevention layer disclosed in Japanese Laid-Open Patent Publication No. 10-239671 mentioned above.
The wavelength of the ultraviolet rays selectively attenuated by the dielectric layer is set depending on the plasma channels used. For example, it is set depending on the conditions of sealed discharge gas such as the kind and/or the pressure thereof. The wavelength of the ultraviolet rays allowed to pass through the dielectric layer may be appropriately set depending on the ultraviolet rays used in the fabrication process of the plasma addressed LCD device.
The expression of xe2x80x9cattenuating ultraviolet raysxe2x80x9d as used herein means attenuating the intensity of incident ultraviolet rays by xe2x80x9cabsorbingxe2x80x9d or xe2x80x9cscatteringxe2x80x9d the ultraviolet rays.
The dielectric layer may be formed of a single dielectric sheet having a property of selectively attenuating the ultraviolet rays. By adopting the dielectric layer formed of a single dielectric sheet having a property of selectively attenuating the ultraviolet rays emitted from the plasma channels, no separate ultraviolet-blocking layer is required for selective ultraviolet attenuation. This simplifies the fabrication process, that is, reduces the number of fabrication steps. This also prevents possible occurrence of peeling off at the interface between such a separate ultraviolet-blocking layer and the dielectric sheet. As a result, a reliable plasma addressed LCD device is provided.
The dielectric layer may include a dielectric sheet transmitting the ultraviolet rays and an ultraviolet-blocking layer formed on at least one surface of the dielectric sheet, and the ultraviolet-blocking layer may have a property of selectively attenuating the ultraviolet rays. Some ultraviolet-blocking materials may not be easily processed into a single dielectric sheet. Such ultraviolet-blocking materials are made usable by adopting the dielectric layer including a dielectric sheet such as a glass sheet transmitting the ultraviolet rays and an ultraviolet-blocking layer having a property of selectively attenuating the ultraviolet rays. Naturally, ultraviolet-blocking materials that can be easily processed into a single dielectric sheet may be used.
Preferably, the ultraviolet-blocking layer includes inorganic particulates that attenuate the ultraviolet rays. By forming the ultraviolet-blocking layer including inorganic particulates that attenuate the ultraviolet rays emitted from the plasma channels, selection of the wavelength of ultraviolet rays to be attenuated is easy. That is, the band gap of inorganic particulates can be easily controlled by appropriately selecting the compound. It is therefore possible to control the properties of the ultraviolet-blocking layer so that the layer selectively attenuates ultraviolet rays having a desired wavelength while transmitting ultraviolet rays in other wavelength ranges. Since the ultraviolet-blocking layer including such inorganic particulates is formed on the dielectric sheet made of sheet glass, it also serves as a hard coat layer protecting the dielectric sheet from being damaged in the process of bonding a plasma channel-side substrate with a color filter-side substrate, the process of injecting a liquid crystal material, and the like.
The ultraviolet-blocking layer may include an ultraviolet absorbent and/or a photostabilizer. When an organic material is used to form the ultraviolet-blocking layer, in particular, both an ultraviolet absorbent and a photostabilizer are preferably used. If only the ultraviolet absorbent is used, degradation of the liquid crystal molecules or the alignment layers may not be sufficiently prevented. To state in more detail, various radicals are produced by absorption of ultraviolet rays by the ultraviolet absorbent. The produced radicals may react with the organic materials such as the liquid crystal molecules and the alignment layers constituting the liquid crystal cell, possibly resulting in degrading the organic materials. The photostabilizer captures the produced radicals, and thus can prevent the organic materials constituting the liquid crystal cell from reacting with the radicals and thus degrading. The ultraviolet-blocking layer including the ultraviolet absorbent and the photostabilizer serves as a hard coat layer preventing damage of the dielectric layer, as the ultraviolet-blocking layer including inorganic particulates does as described above. The voltage applied between the electrodes of the liquid crystal cell and the plasma channels is capacitance-divided. A divided voltage is therefore applied across the liquid crystal layer of the plasma addressed LCD device. By the addition of the ultraviolet-blocking layer between the plasma channels and the liquid crystal layer, the voltage applied across the liquid crystal layer is reduced. If the voltage is reduced to 95% or less of the case having no ultraviolet-blocking layer, decrease in contrast ratio is eminent. Preferably, Zf is 20% or less of Zg when Zf is df/∈f and Zg is dg/∈g where df and ∈f are the thickness and the dielectric constant of the ultraviolet-blocking layer, and dg and ∈g are the thickness and the dielectric constant of the dielectric sheet. By this setting, the voltage applied across the liquid crystal layer is prevented from decreasing to 95% or less of the case having no ultraviolet-blocking layer. Thus, reduction in display brightness and contrast ratio can be suppressed or prevented. Typically, the thickness of the ultraviolet-blocking layer df is 3 xcexcm or less. By this setting, a sufficient voltage can be applied across the liquid crystal layer.
The ultraviolet transmittance of the dielectric layer is preferably 70% or less for a wavelength range of 340 nm or less and 80% or more for a wavelength of 365 nm.
Preferably, the liquid crystal layer includes liquid crystal molecules and a cured ultravioletcurable resin, and the initial orientation of the liquid crystal molecules is stabilized by the cured ultravioletcurable resin.
Further preferably, the LCD device further includes a wall structure on the surface of the substrate facing the liquid crystal layer, the liquid crystal layer is divided into a plurality of liquid crystal regions by the wall structure, and the liquid crystal molecules in the liquid crystal regions are axially symmetrically oriented. In the ASM mode LCD device that further includes such a wall structure on the surface of the substrate facing the liquid crystal layer, the refractive index anisotropy of the liquid crystal molecules is averaged over all the azimuthal directions. This solves the conventional problem that the viewing angle characteristics greatly differ depending on the azimuthal direction, which is observed in the gray scale display in the conventional twisted nematic (TN) mode LCD device. The resultant LCD device has wide viewing angle characteristics. The dielectric layer of the LCD device of the invention sufficiently transmits ultraviolet rays in a specific wavelength as described above. It is therefore possible to execute ultraviolet radiation via the plasma cells for stabilizing initial axially symmetrical orientation of the liquid crystal molecules in the fabrication process of the ASM mode plasma addressed LCD device.
Ultraviolet radiation may also be performed, not only in the fabrication process of the ASM mode plasma addressed LCD device described above, in a process after cell assembly, such as a process of changing the pretilt angle by irradiating an alignment layer with ultraviolet rays (see Japanese Laid-Open Patent Publication No. 10-148835, for example).
The method for fabricating the liquid crystal display device of the first embodiment of the invention is a method for fabricating a liquid crystal display device including: a substrate; a dielectric layer; a liquid crystal layer sandwiched by the substrate and the dielectric layer; a plurality of stripe-shaped electrodes formed on a surface of the substrate facing the liquid crystal layer to extend in parallel with a first direction; and a plurality of stripe-shaped plasma channels formed to face the plurality of electrodes with the liquid crystal layer and the dielectric layer therebetween to extend in parallel with a second direction different from the first direction, a plurality of pixel regions being formed in respective crossings of the plurality of electrodes and the plurality of plasma channels, the device further comprising a wall structure on the surface of the substrate facing the liquid crystal layer, the liquid crystal layer being divided into a plurality of liquid crystal regions by the wall structure, the liquid crystal molecules in the liquid crystal regions being axially symmetrically oriented. The method includes the steps of: forming the wall structure on the substrate; preparing the dielectric layer that selectively attenuates ultraviolet rays emitted from the plurality of plasma channels; injecting a material including liquid crystal molecules and an ultravioletcurable resin in a space between the substrate on which the wall structure is formed and the dielectric layer; and stabilizing initial orientation of the liquid crystal molecules by irradiating the material with ultraviolet rays having a wavelength of 365 nm via the dielectric layer to cure the ultravioletcurable resin.
According to the above method, a material including liquid crystal molecules and an ultravioletcurable resin is injected into a space between the substrate on which the wall structure is formed and the dielectric layer. Then, the material is irradiated with ultraviolet rays having a wavelength of 365 nm via the dielectric layer to cure the ultravioletcurable resin, whereby initial orientation of the liquid crystal molecules is stabilized. As a result, it is possible to fabricate an ASM mode LCD device that can prevent deterioration in the display quality of the liquid crystal cell due to ultraviolet radiation during use. In particular, the present invention is effective for plasma addressed LCD devices for color display for the following reason. A color plasma addressed LCD device includes a color filter layer formed on the substrate of the liquid crystal cell. The color filter layer generally absorbs ultraviolet rays, and thus blocks sufficiently intense ultraviolet rays from entering the liquid crystal layer via the liquid crystal cell. Therefore, for fabrication of an LCD device that necessitates ultraviolet radiation to the liquid crystal layer, such as an ASM mode color plasma addressed LCD device, the method according to the present invention is advantageous because ultraviolet radiation to the liquid crystal layer is possible via the dielectric layer on the side of the plasma cell.
Preferably, the step of preparing the dielectric layer includes the steps of: preparing a dielectric sheet having a property of selectively attenuating the ultraviolet rays; and chemically abrading the dielectric sheet. The dielectric sheet having a property of selectively attenuating the ultraviolet rays may be formed by first forming a comparatively thick dielectric sheet and then chemically abrading the dielectric sheet. This fabrication method alleviates the limitations on the material and the formation method for the dielectric sheet having a property of selectively attenuating the ultraviolet rays, and thus permits formation of a dielectric sheet with more excellent properties.
The liquid crystal display device of the second embodiment of the invention includes: a substrate; a dielectric layer; a liquid crystal layer sandwiched by the substrate and the dielectric layer; a plurality of stripe-shaped electrodes formed on a surface of the substrate facing the liquid crystal layer to extend in parallel with a first direction; and a plurality of stripe-shaped plasma channels formed to face the plurality of electrodes with the liquid crystal layer and the dielectric layer therebetween to extend in parallel with a second direction different from the first direction, a plurality of pixel regions being formed in respective crossings of the plurality of electrodes and the plurality of plasma channels, wherein the device further includes a pair of alignment layers formed on both surfaces of the liquid crystal layer, at least the alignment layer of the pair of alignment layers formed on the side of the dielectric layer selectively attenuates ultraviolet rays emitted from the plurality of plasma channels, and has an ultraviolet transmittance of less than 70% for a wavelength range of 340 nm or less.
The LCD device of the second embodiment of the invention includes a pair of alignment layers formed on both surfaces of the liquid crystal layer. At least the one of the pair of alignment layers formed on the side of the dielectric layer selectively attenuates ultraviolet rays emitted from the plurality of plasma channels. Accordingly, the alignment layer suppresses or prevents the organic materials such as liquid crystal molecules and the alignment layer itself constituting a liquid crystal cell from degrading due to ultraviolet rays of a specific wavelength generated by plasma discharge during use, and yet can transmit ultraviolet rays in other ranges. It is therefore possible to perform aligning processing and the like using ultraviolet rays allowed to pass through the dielectric layer. The wavelength of the ultraviolet rays selectively attenuated by the alignment layer is set depending on the plasma channels. The wavelength of the ultraviolet rays allowed to pass through the alignment layer may be appropriately set depending on the ultraviolet rays used in the fabrication process of the plasma addressed LCD device.
By adopting the alignment layer having a property of selectively attenuating the ultraviolet rays emitted from the plasma channels, no separate ultraviolet-blocking layer is required for selective ultraviolet attenuation. This simplifies the fabrication process, that is, reduces the number of fabrication steps. This also prevents possible occurrence of peeling off at the interface between such a separate ultraviolet-blocking layer and the alignment layer, and thus a reliable plasma addressed LCD device is obtained.
The ultraviolet transmittance of the alignment layer is preferably 40% or less for a wavelength of 320 nm and 80% or more for a wavelength of 365 nm.
Preferably, the liquid crystal layer includes liquid crystal molecules and a cured ultravioletcurable resin, and the initial orientation of the liquid crystal molecules is stabilized by the cured ultravioletcurable resin. By this stabilization, disorder of the orientation of the liquid crystal molecules is avoided, and thus variation in viewing angle characteristics during display is suppressed.
Preferably, the LCD device further includes a wall structure on the surface of the substrate facing the liquid crystal layer, the liquid crystal layer is divided into a plurality of liquid crystal regions by the wall structure, and the liquid crystal molecules in the liquid crystal regions are axially symmetrically oriented. The resultant ASM mode LCD device has wide viewing angle characteristics. Also, ultraviolet radiation is possible via the plasma cell in the fabrication process.
The alignment layer preferably includes a polymer material and inorganic particulates that attenuate the ultraviolet rays. By using the alignment layer including inorganic particulates, selection of the wavelength of ultraviolet rays to be attenuated is easy.
The alignment layer preferably includes a polymer material and an ultraviolet absorbent. Preferably, it further includes a photostabilizer. The alignment layer including an ultraviolet absorbent can absorb ultraviolet rays incident on the alignment layer. The alignment layer that further includes a photostabilizer can effectively capture radicals produced from the ultraviolet absorbent. Accordingly, by use is of an ultraviolet absorbent, and by use of a photostabilizer additionally, degradation of the liquid crystal cell is prevented.
The volume resistivity of the alignment layer is preferably 5xc3x971012xcexa9xc2x7cm or more. By setting the volume resistivity of the alignment layer at 5xc3x971012xcexa9xc2x7cm or more, the voltage retention can be kept high. Specifically, in order to keep the voltage retention high, the current flowing to the liquid crystal layer needs to be as small as possible. The specific resistance of a liquid crystal material is normally 1xc3x971012xcexa9xc2x7cm or more, and the current flowing to the alignment layer that is in contact with the liquid crystal layer needs to be as small as possible. In consideration of these, the volume resistivity of the alignment layer is preferably greater than 1xc3x971012xcexa9xc2x7cm, more preferably 5xc3x971012xcexa9xc2x7cm or more.
The method for fabricating the liquid crystal display device of the second embodiment of the invention is a method for fabricating a liquid crystal display device including: a substrate; a dielectric layer; a liquid crystal layer sandwiched by the substrate and the dielectric layer; a plurality of stripe-shaped electrodes formed on a surface of the substrate facing the liquid crystal layer to extend in parallel with a first direction; and a plurality of stripe-shaped plasma channels formed to face the plurality of electrodes with the liquid crystal layer and the dielectric layer therebetween to extend in parallel with a second direction different from the first direction, a plurality of pixel regions being formed in respective crossings of the plurality of electrodes and the plurality of plasma channels. The method includes the steps of: forming an alignment layer on the dielectric layer, the alignment layer selectively attenuating ultraviolet rays emitted from the plurality of plasma channels and having an ultraviolet transmittance of less than 70% for a wavelength range of 340 nm or less; and performing aligning processing for the alignment layer.
The above fabrication method includes the steps of forming an alignment layer on the dielectric layer for selectively attenuating ultraviolet rays emitted from the plurality of plasma channels and performing aligning processing for the alignment layer. By these steps, prevented are deterioration in the display quality of the liquid crystal cell due to ultraviolet radiation and disorder of the orientation of the liquid crystal molecules. As a result, it is possible to fabricate an LCD device that prevents deterioration in the display quality of the liquid crystal cell due to ultraviolet radiation during use.
As the method for fabricating another liquid crystal display device of the second embodiment of the invention in which liquid crystal molecules are axially symmetrically oriented, provided is a method for fabricating a liquid crystal display device including: a substrate; a dielectric layer; a liquid crystal layer sandwiched by the substrate and the dielectric layer; a plurality of stripe-shaped electrodes formed on a surface of the substrate facing the liquid crystal layer to extend in parallel with a first direction; and a plurality of stripe-shaped plasma channels formed to face the plurality of electrodes with the liquid crystal layer and the dielectric layer therebetween to extend in parallel with a second direction different from the first direction, a plurality of pixel regions being formed in respective crossings of the plurality of electrodes and the plurality of plasma channels, the device further comprising a wall structure on the surface of the substrate facing the liquid crystal layer, the liquid crystal layer being divided into a plurality of liquid crystal regions by the wall structure, the liquid crystal molecules in the liquid crystal regions being axially symmetrically oriented, the method comprising the steps of: forming the wall structure on the substrate; forming an alignment layer on the dielectric layer, the alignment layer selectively attenuating ultraviolet rays emitted from the plurality of plasma channels and having an ultraviolet transmittance of less than 70% for a wavelength range of 340 nm or less; injecting a material including liquid crystal molecules and an ultravioletcurable resin in a space between the dielectric layer on which the alignment layer is formed and the substrate on which the wall structure is formed; and stabilizing initial orientation of the liquid crystal molecules by irradiating the material with ultraviolet rays having a wavelength of 365 nm via the dielectric layer to cure the ultravioletcurable resin.
According to the above method of the invention, initial orientation of the liquid crystal molecules is stabilized. As a result, it is possible to fabricate an ASM mode LCD device that prevents deterioration in the display quality of the liquid crystal cell due to ultraviolet radiation during use. Accordingly, for fabrication of an LCD device that necessitates ultraviolet radiation to the liquid crystal layer, such as an ASM mode color plasma addressed LCD device, the method according to the invention is advantageous because ultraviolet radiation to the liquid crystal layer is possible via the dielectric layer on the side of the plasma cell.